Rolled steel having high strength and low impact transition temperature and method of producing same

ABSTRACT

Rolled steels characterized in their as-rolled condition by an unexpected combination of high strength and low impact transition temperature. The product is produced by rolling a steel workpiece containing both austenite and ferrite, preventing complete recrystallization at any time thereafter, and continuing the rolling of said workpiece as it is cooling in the temperature range between the Ar1 temperature and 600*F.

U United States Patent 1191 1111 3,753,796 Melloy et al. Aug. 21, 1973 1 ROLLED STEEL HAVING HIGH STRENGTH 2,857,299 10/1958 Epstein et a1. 148/124 AND ow IMPACT TRANSITION 3,207,637 9/1965 Matuschka et al. 148/124 3,228,808 l/1966 Ripling et a1. 148/124 TEMPERATURE AND METHOD OF 3,386,862 6/1968 Johnston et a] PRODUCING SAME 3,539,404 11/1970 DeRetana 148/124 75 Inventors; George R Melloy; Joseph 3,388,988 6/1968 Nagashima et a]. 148/123 Dennison both of Bethlehem, Pa 2,108,588 2/1938 Lawrence 148/12 [73] Assignee: Bethlehem Steel Corporation,

Bethlehem, Pa. Primary Examiner-- W. W. Stallard June 2 Attorney-Joseph O'Keefe [21] App1.No.: 156,215

Related [1.8. Application Data [60] Division of Ser. No. 786,844, Dec. 20, 1968, Pat. No. [57] ABSTRACT 3,645,801, which is a continuation-in-part of Ser. No. 741,372, June 27, 1968, abandoned. Rolled steels characterized in their as-rolled condition 1 by an unexpected combination of high strength and low [52] US. Cl. 148/36 impact transition temperature. The product is pro [51] Int. Cl C2ld 9/46 duced by rolling a steel workpiece containing both aus- [58] Field of Search 148/12, 12.1, 12.3, tenite and ferrite, preventing complete recrystallization 148/ 12.4, 36 at any time thereafter, and continuing the rolling of t said workpiece as it is cooling in the temperature range 6] References Cited between the Ar, temperature and 600F.

UNITED STATES PATENTS 2,858,206 /1958 Boyce et al. 148/124 4 Claims, 3 Drawing Figures l f 1 i :1. I I I +|oo i g 1 LOW-FINISHING A 1 1 2 TEMPERATURE ROLLING A v 1- l 1 2 mg v 9 l o nor D 1 2 ROLLING, F q 1 1 n: 1 A5 I Q 1 13- i 1 5 i 5 g J l I 1 L t i -1oo i 1 e L U i '9 1 6 A. ML x I N v I CONTINUUM I ROLLING 1 Ln M ..2o0 W- E,.. .UV. Y .V Q M 1 i s l 50 6O 7O 8O HO YIELD POINT OR YIELD STRENGTH 28.1. (Thousands) V-l CHARPY IMPACT TRANSITION TEMPERATURE F I I G 6 O O PATENTEBMJBZI IIIII SHEET 3 [1? Fig. 3

I H I LOW-FINISHING M TEMPERATURE-ROLLING I I ca I G l B /E}- .D HOT ROLLING AE I Q I I I l JVQ I l l N I CONTINUUM ROLLING l L 1M V I 7o no YIELD POINT OR YIELD STRENGTH RSJ. (Thousands) INVENTORS George F. Melloy Joseph D. Dennison ROLLED STEEL HAVING HIGH STRENGTH AND LOW IMPACT TRANSITION TEMPERATURE AND METHOD OF PRODUCING SAME CROSS-REFERENCE TO RELATED APPLICATIONS This application is a division of U. S. Pat. application Ser. No. 786,844, filed Dec. 20, 1968, now US. Pat. No. 3,645,801, which is a continuation-in-part of application Ser. No. 741,372, filed June 27, 1968 now abandoned.

BACKGROUND OF THE INVENTION This invention is directed to rolled steel and a method of producing the same. For convenience, the method is sometimes hereinafter called continuum rolling.

Normal practice used to produce hot-rolled steel consists of heating the steel to a temperatureat which it is fully austenitic and then reducing the steel to final.

dimensions as rapidly as possible with a minimum of heat loss, generally finishing above the Ar temperature. Suchpractice takes advantage of ease of working at high temperature, but does not develop the combination of desirable properties possible of development in as-rolled steel.

Prior studies have shown that higher strength can be developed in the steel by use of low-finishingtemperature rolling. In this practice, the steel is heated and partially reduced while austenitic, as in hot rolling. Further reduction of the cross-section is then delayed to allow the workpiece to cool to a lower-than-normal temperature after which rolling is continued to the final desired cross-section in one or more passes.

In these prior art practices, the cross-sectional area of the workpiece is reduced tofinal size and shape in a sequence of rolling passes. Such passes may include twogeneral types, those intended mainly to reduce the cross-sectional area of the workpiece and those intended mainly for gage orshape and which may be accompanied by relatively little reduction. As a matter of cal. Further, it is known that development of the aforementioned higher strength by low-finishingtemperature rolling requires reduction of crosssectional area of approximately 15 percent or more at the lower-than-norrnal temperature. 1

In low-finishing-temperature rolling, it is known that the steel is strengthened and its impact transition temperature is decreased by lowering the finishing temperature below the Ar, temperature. It is also known that further strengthening is obtained by further lowering the finishing temperature below the Ar temperature, but impact transition temperature is thereby increased. In contrast, we have found that by use of continuum rolling, such further strengthening is accompanied by a decrease rather than an increase in impact transition temperature. This combination of high strength and low impact transition temperature is an unexpected and very desirable combination of properties.

SUMMARY OF THE INVENTION The steels of this inventionare characterized in their as-rolled condition (a) by having greater strengths and lower impact transition temperatures than steels of the same composition produced by hot rolling and (b) by having lower impact transition temperatures than steels of the same composition produced to the same strength by low-finishing-temperature rolling.

Broadly, the method of this invention comprises:

1. providing a steel workpiece at a tempertaure at which it is essentially completely austenitic,

2. rolling said workpiece as it is cooling in the temperature range between the Ar; and Ar temperatures,

3. continuing the rolling of said workpiece as it is cooling in the temperature range between the Ar temperature and 600 F., and

4. preventing complete recrystallization at any time after completion of step 2.

The benefits of the invention are primarily applicable to steels containing not more than 0.35 percentcarbon and not more than a total of 3 percent of other elements other than iron.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing the relationship between typical cooling curves of steel plates and curves representing various percentages of recrystallization.

FIG. 2 shows a possible continuum rolling sequence superimposed on a recrystallization diagram similar to FIG. 1.

FIG. 3 is a graph comparing impact transition temperatures and yield points or yield strengths of steel of a particular composition produced by continuum rolling with those of steel of the same composition produced by l (1) hot rolling and (2) low-finishingtemperature rolling.

DESCRIPTION OF THE. PREFERRED EMBODIMENTS The method of this invention differs from prior art methods in that (a) the steelmust be rolled in two temperature ranges, namely, between the Ar; and the Ar, temperatures and between the Ar, temperature and 600 F. and preferably in three temperature ranges, namely, the two first mentioned temperature ranges and the hot-rolling temperature range and (b) complete recrystallization at any time after completion of the rolling in the Ar; Ar, temperature range must be avoided. t

The workpiece for rolling in the Ar, Ar temperature range is a steel workpiece at a temperature at which it is essentially completely austenitic. Various methods of obtaining said workpiece will be recognized by those skilled in the art but preferably said workpiece is obtained by hot rolling. The amount of reduction in suchhot rolling is not critical, but from a practical viewpoint it is helpful to take as much reduction as possible because of ease of working. Said rolling in the Ar, Ar, temperature range must becarried out as the. workpiece is cooling in that range and;

must comprise one or more rolling passes including at least one reduction pass. The meaning of the term re.- duction pass, as used herein, encompasses both a single pass in which the reduction of the cross-sectional area of the workpiece is at least 15 percent and two or more passes effecting an equal amount of reduction and taken in rapid enough succession to prevent any recrystallization between passes.

Following the rolling in the Ar Ar temperature range, rolling must be continued as the workpiece is cooling in the Ar 600 F. temperature range and must comprise one or more rolling passes in that range including at least one reduction pass.

Conditions following the rolling in the Ar, Ar temperature range must be such that complete recrystallization of the deformed grains produced by the last reduction pass in said temperature range does not take place at any time after such last reduction pass and preferably such that percentage recrystallization does not exceed 60 percent at any time after such last reduction pass.

As used herein, percentage recrystallization means the cross-sectional area of recrystallized grains expressed as a percentage of total cross-sectional area on a plane transverse to the direction of maximum elongation of the workpiece during rolling.

FIG. 1 illustrates diagrammatically the principles upon which reduction in the Ar Ar range and in the Ar 600 F. range depend. In the FIGURE, the abscissa represents time on a logarithmic scale with zero time representing the time of completion of a reduction pass. The ordinate represents temperature, with the Ar, and Ar temperatures indicated. Lines 1, 2, and 3 rep resent, respectively, times and temperatures for commencement of, 60 percent, and 100 percent recrystallization. Lines 4, 5, 6, 7 and 8 represent the cooling of steel workpieces. It will be understood that the values shown in the FIGURE are illustrative only; actual values depend on the actual workpieces being considered and can be determined by known methods.

As stated hereinabove, it is necessary to roll the workpiece in the Ar; Ar, range and to follow such rolling by rolling in the Ar, 600 F. range while maintaining conditions such that complete recrystallization is avoided and preferably such that recrystallization does not exceed 60 percent at any time after completion of the last reduction pass in the first mentioned rolling. In the FIGURE, Point A on Line 8 indicates a temperature to which a steel workpiece has cooled and which is within the Ar, Ar range. Line represents the cooling of said workpiece after it has been given a reduction pass at the temperature represented by Point A. It will be seen that as said reduced workpiece cools along Line 5, it completely recrystallizes before it cools to a temperature below the Ar temperature, thus becoming unsuitable for the subsequent reduction in the Ar 600 F. range. There are ways of meeting this situation. Oneway is to accelerate the cooling of said reduced workpiece by water sprays, air blasts or the like so that it cools to a temperature below the Ar temperature while avoiding complete recrsystallization, as shown for example by Line 4. A second way is to allow said reduced workpiece to cool as shown by Line 5 to a temperature as indicated for example by Point B and then to give it a second reduction pass within the Ar, Ar range. As shown by Line 6, such twice-reduced workpiece cools to a temperature below the Ar, temperature with less than 60 percent recrystallization. A third way is to allow the aforementioned workpiece of Line 8 to cool to a lower temperature, as indicated for example by Point C, and then to give it a reduction pass within the Ar, Ar range. The cooling of such reduced workpiece is also shown by Line 6.

As stated hereinabove, complete recrystallization of the steel at any time after the last reduction pass in the Ar;, Ar range must be prevented. In FIG. 1, Line 7 represents the cooling of a steel workpiece after a reduction pass at a temperature below the Ar, temperature. It is seen that recrystallization is avoided as such reduced workpiece cools. However, if the temperature at which a reduction pass is effected and the cooling and recrystallization characteristics of the resulting reduced workpiece are such that it would recrystallize before being given another reduction pass or before cooling to a temperature sufficiently low to prevent recrystallization, then the cooling of said reduced workpiece must be accelerated by water sprays, air blasts or the like so as to prevent complete recrystallization and preferably so as to prevent exceeding 60 percent recrystallization.

FIG. 2 illustrates diagrammatically a possible rolling sequence for producing a A inch plate by continuum rolling. In the FIGURE the abscissa represents time on a logarithmic scale, with zero time representing the time at completion of a reduction pass. The ordinate represents temperature on an arithmetic scale, with Ar;, and Ar temperatures indicated. Lines 1-3 represent times and temperatures for commencement of, 60 percent, and percent recrystallization. The horizontal dashed lines represent instantaneous return to zero time. In this example, a 2% inch slab has been rolled from a 4 inch slab at temperatures well above the Ar, temperature and, as shown in the FIGURE, has air cooled in about 2 minutes to about l,720 F. This 2% inch slab is given two passes at temperatures above the Ar; temperature, first 02% inch (A) and then to 1% inch (B). After each of these passes the plate cools in air for about 2 minutes, cooling during the latter period to about l,385 F., somewhat below the Ar; temperature.

In the Ar, Ar, temperature range the steel is given two passes, first to 1% inch (C) and then to 1 inch (D). After each of these passes the plate cools in air for about 2 minutes, cooling during the latter period to about 1,l60 F., somewhat below the Ar, temperature. Percentage recrystallization following the first pass is small, and there is no recrystallization following the second pass.

In the temperature range between the Ar, temperature and 600 F. the steel is given three passes, first to inch (B), then to "it; inch (F), and finally to '15 inch (G). After each of the first two of these passes the plate cools in air for about 2 minutes; after the final pass, the plate cools in air to room temperature. There is no recrystallization following any of these passes. i

All of the passes in this rolling sequence are reduction passes and it is to be understood that such other Mn P S 1.03.01 .015

balance substantially iron.

4 inch thick slabs of this steel were heated to a temperature of 2,200 F. and rolled to '75 inch plate in accordance with the following schedule:

Thickness After Reduction in Pass No. Each Pass Thickness 4 2%" 12.5% 5 2%" l9.l% 6 1%" 17.6% 7 1%" 21.4% 8 1" 27.2% 9 #1" 25.0% l0 W 16.7% ll Vt" 20.0%

Box pass to obtain width.

The Ar and Ar temperatures of this steel were l,500 F. and l,320 F. respectively at a cooling rate of approximately 1,000 F./hr.

Table 1 shows the temperature ranges in which they Table 2 shows the finishing temperature, yield point or yield strength, tensile strength, elongation, and V-l5 Charpy impact transition temperature of each of the thirteen plates A-N.

FIG. 3 shows for the said thirteen plates the manner in which impact transition temperature varies with strength as dependent on the method of rolling. In the FIGURE, points A, B, and C represent plates produced byhot rolling, points D and E represent plates finished at temperatures between the ,Ar and the Ar, temperatures by low-finishing-temperature rolling, points F, G, and H represent plates finished at temperatures below the Ar, temperature by low-finishingtemperature rolling, and points J, K, L, M, and N represent plates produced by continuum rolling. Both the impact transition temperature of approximately plus 80 F., whereas continuum rolling could provide a plate of the same strength with an impact transition temperature nearly 200 lower, namely, approximately minus 115 F.

Table 3 gives the percentage compositions of seven other steels from which specific examples of plates were produced by continuum rolling and by prior art methods. In each of these steels the balance of the composition was substantially iron. Varieties included are semi-killed, killed, carbon, low alloy, bainitic, and precipitation hardening steels.

The Ar and Ar, temperatures of the Table 3 steels at cooling rates of approximately 1,000 F./hr. are shown in Table 4.

TABLE 4 NO. Ar, Ar,

1 1520 F. l400 F. 2 1520" F. 1320" F. 3 1500 F. 1300 F. 4 [440 F. 1220 F. 5 1400" F. 1200 F. 6 1400 F. 1 160 F. 7 1 140 F. 960 F.

One or more slabs of each of the compositions specitied in Table 3 were rolled to '16 inch plate by continuum rolling. For comparative purposes, one or more slabs of each composition were also rolled into 1% inch plate by prior art rolling methods. The details of the rolling practices, and the resulting yield strengths or yield points, tensile strengths, and V-l5 Charpy impact transition temperatures are set forth in Tables 5-ll.

In reference to the data set forth in Tables 5-11, it is well known that the strength and impact transition temperature of a steel are affected both by the composition of the steel and by the way in which it is processed. For example, Tables 5-11 show a wide range of strengths and impact transition temperatures for steels of seven different compositions each of which was rolled by 0 prior art methods and by one or more examples of con- FIGURE and Table 2 show clearly that the plates produced by continuum rolling have higher strengths and low'erimpact transition temperatures than do those produced by hot rolling. Moreover, Table 2 shows clearly that finishing the rolling at temperatures below 'the Ar temperature either by low-finishingtemperature rolling or by continuum rolling increased the strength of the steel over that obtained by finishing the rolling at temperatures above the Ar temperature. However, the important distinction shown by both the TABLE, and the FIGURE is that when such further strengthening is by low-finishing-temperature rolling impact transition temperature is raised. as strength is increased, whereas when suchfurther strengthing is by continuum rolling impact transition temperature is lowered as strength is increased. The difference is substantial. For example, the dashed lines on the FIGURE show that low-finishing-temperature rolling could provide a plate with a yield'strength of 75,000 psi and an tinuum rolling, while Table 2 shows a wide range of strengths and impact transition temperatures for a steel of a particular composition rolled in a number of ways, including several different examples of continuum rolling. Every plateproduced by continuum rolling has an unexpectedly good combination of strength and impact transition temperature, but for the reason cited above it is not possible to broadly characterize the product of continuum rolling in terms of specific values of those properties. As said earlier, the products of continuum rolling are characterized in their as-rolled condition (a) by having greater strengths and lower impact transition temperatures than steels of the same composition produced by hot-rolling and (b) by having lower impact transition temperatures than steels of the same compositions produced to the same strength by low-finishingtemperature rolling.

Although the method of this invention hasbeen described only in connection with the rolling of plates, the method may be used in the rolling of other products such as billets, bars, structural sections and the like.

All references herein to steel compositions in terms of percentages are weight percentages.

TABLE 7 Steel No. 3 Low C-Mn-V-N Reduc- Yield Tensile V-i5 Charpy No. oi Temperature Thickness, tion, point, strength, transition Rcduetionpraetice passes range, F. inches percent ins. k.s.i. temperature Hot roiling. 11 2, 200-1, 610 4--+%" 87. 5 49. 3 61. 25 F o i0 2, 200-1, 505 4-' 87.5 55.0 62. 1 -55 F. Continuum rolling 0 2, 200-1, 500 4"-1 D0 2 1,500-1,300 1%- 1" D0 2 l,300-1,205 1 -185 F.

Yield strength (0.2% oflset).

TABLE 8 Steel No. 4 Low Alloy Reduc- Yield Tensile V-15 Charpy No. of Temperature Thickness, tion, point, strength, transition Reduction practice passes range, F. inches percent k.s.i. k.s.i. temperature Hot rolling 11 2, 200-1, 700 5-%" 90.0 51.7 72. 5 F. D 11 2, 200-1, 490 5"%" 90.0 56. 5 73. 5 70 F.

" Yield strength (0.2% offset).

M u m; 0

mm N, rm-M" (mum,

Tensile V-l5 Uharpy Redue, Yieid No. of Temperature Thickness, tion, point, strength, transition Reduction practice passes range, F. inches percent k.s.i. k.s.i. temperature Yield strength (0.2% oflset).

" TABLE in Steel No. 6 1% Copper Reduc- Yield Tensile V-15 Cilarpy No. of Temperature Thickness, tion, point, strength, transition Reduction practice passes range, F. inches percent k.s. k.s.i. temperature Hot rolling 11 2, 200-1, 650 4-- Continuum rollin 5 2, ZOO-1,400 4" 2 Do 3 l,400-l,160 2%---1" Do 3 1.160-040 1" )5" l F.

Yield strength (0.2% oilset).

TABLE i 1 Steel No. 7 iialnitl';

Reduc- Yield 'leneii V-i5 (,harpy Reduction practice 2; Tegzxpggaguf e Thiciknrehsgs, tion, point, strength, transition percent k.s. k.s.i temperature :3 g, ZOO-1,500 4::--%:: 87.5 41.1 79. 2 '5 F. eerie ....e- 2 1, -960 1%"-.830" 39. 6 1 920 .830-, 39. 8 '08. 4 115. 0 205 F.

Yield strength (0.2% oflset).

We claim: which have been recrystallized by an amount not to ex- A ferrous alloy article which has been heated then d 60 percent Worked as it is cooled through the range between the 2. The ferrous alloy article of claim 1 wherein said Ar and Ar temperatures, and subsequently between article is a rolled steel plate g gg i fig ii g; a 5 3. The ferrous alloy article of claim 1 characterized arpy lmpac fans on empera we no lg er by a V-l5 Charpy impact transition temperature not than about 50 F and by a yield strength of at least o 65,000 psi and containing, by weight percent, a maxig s ggg l loo F and by a yleld strength of at least mum of 0.35 percent carbon, at maximum total of less l than 3.0 percent of other elements, and the balance The ferrous alloy amcle of clam 3 wherein Sald iron, and a crystalline structure of deformed grains article iS 8 o ed Steel plate. 

2. The ferrous alloy article of claim 1 wherein said article is a rolled steel plate.
 3. The ferrous alloy article of claim 1 characterized by a V-15 Charpy impact transition temperature not higher than -100* F and by a yield strength of at least 75,000 psi.
 4. The ferrous alloy article of claim 3 wherein said article is a rolled steel plate. 